1. Field of the Invention
The present invention relates to a holographic recording composition suited to the manufacturing of a holographic recording medium, and more particularly, a volume holographic recording medium. The present invention further relates to a holographic recording medium formed with the above holographic recording composition.
2. Discussion of the Background
Holographic optical recording media based on the principle of the holograph have been developed. Recording of information on holographic optical recording media is carried out by superposing an informing light containing image information and a reference light in a recording layer comprised of a photosensitive composition to write an interference fringe thus formed in the recording layer. During the reproduction of information, a reference light is directed at a prescribed angle into the recording layer in which the information has been recorded, causing optical diffraction of the reference light by the interference fringe which has been formed, reproducing the informing light. For example, Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2005-502918 or English language family member WO 03/023519, which are expressly incorporated herein by reference in their entirety, disclose the use of a urethane matrix and a phenyl acrylate derivative in a holographic optical recording medium of the photopolymer type.
In recent years, volume holography, and, more particularly, digital volume holography, have been developed to practical levels for ultrahigh-density optical recording and have been garnering attention. Volume holography is a method of writing interference fringes three-dimensionally by also actively utilizing the direction of thickness of an optical recording medium. It is advantageous in that increasing the thickness permits greater diffraction efficiency and multiplexed recording increases the recording capacity. Digital volume holography is a computer-oriented holographic recording method in which the image data being recorded are limited to a binary digital pattern while employing a recording medium and recording system similar to those of volume holography. In digital volume holography, for example, image information such as an analog drawing is first digitized and then expanded into two-dimensional digital pattern information, which is recorded as image information. During reproduction, the digital pattern information is read and decoded to restore the original image information, which is displayed. Thus, even when the signal-to-noise (S/N) ratio deteriorates somewhat during reproduction, by conducting differential detection or conducting error correction by encoding the two-dimensional data, it is possible to reproduce the original data in an extremely faithful manner (see Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-311936 or English language family member US 2002/0114027 A1, which are expressly incorporated herein by reference in their entirety).
The above-mentioned volume photographic optical recording medium is an attempt to increase sensitivity. However, the sensitivity of conventional holographic recording media, including that of the holographic recording medium described in Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2005-502918, is not necessarily adequate for volume holographic recording and requires further improvement.
In photopolymer-type holographic optical recording media such as the holographic recording medium described in Published Japanese Translation of a PCT International Application (TOKUHYO) No. 2005-202918, a signal beam and an interference beam are generally irradiated to cause polymerization of a recording monomer, thereby forming an interference image in the recording layer. A fixing beam is then directed onto the recording layer in which the interference image has been formed to fix the interference image. When the recording monomer and radical polymerization initiator remain in large quantities at the completion of the steps of forming and fixing the interference image (the recording reaction), the polymerization reaction progresses further when the recording medium in which the interference image has been formed is exposed to bright light, creating a problem in that the recording is damaged and archiving properties (recording retention) deteriorate. Thus, at the end of the recording reaction, it is desirable for only small quantities of polymerization reaction components in the form of the recording monomer and radical polymerization initiator to remain. The designing of such a system is desirable.